1. Field of the Invention
The present invention relates to a method and an apparatus for producing a dither matrix (threshold matrix) required for performing quantization processing on an multi-tone image, image processing method and apparatus, image forming method and apparatus, a program and a computer readable information recording medium. The present invention is advantageous when it is used in an image forming apparatus using an electrophotographic system such as a laser printer.
2. Description of the Related Art
Image data input to an image forming apparatus such as a copier, a printer or such has multi-tone data of 8 through 12 bits for each pixel in a case where the image data is of a multi-tone image such as that taken by a digital still camera or such. In contrast thereto, in an image forming apparatus which forms an image on paper (so-called ‘hard copy’), the number of tone levels available is substantially very small for each pixel. In order to solve such a problem due to mismatch, a resolution is improved as 600 dpi, 1200 dpi or such, a plurality of pixels are used to modulate an image tone in terms of an area and thus, a halftone image is represented in a pseudo manner, in such a hard copy producing image forming apparatus. In this case, the pseudo halftone processing is performed, which is performed in a process of transforming input image data into a pseudo halftone image. The present invention relates to a dither method which is one type of the pseudo halftone processing method, and, relates to a method for producing a dither matrix (threshold matrix) required for carrying out quantization processing on a multi-tone image.
Since quantization processing of multi-tone image data according to the dither method is known (for example, see ‘Electro-Photographic Society Paper, Vol. 24, No. 1 (1985), pages 51-59’), the details thereof are omitted.
The dither matrix is classified, roughly, into a dot concentration type (dot screen), a Bayer type (diffusion type) and a line screen type. In the present invention, a method of producing a dot concentration type dither matrix is proposed.
The dot concentration type dither matrix has the following advantages. In this type, a dot growth order (an order of pixels actually written for increase an image tone level there) is determined from a pixel which is closer to a predetermined pixel called a growth center and then, to a peripheral pixel, gradually in sequence. Therefore, it is possible to increase an area where dots are overlapped (to shorten a peripheral length of a halftone dot produced by a plurality of dots actually written), in comparison to a case of the Bayer type in which dots are discretely plotted. Generally speaking, a peripheral unit of each dot spreads further than an ideal one in many types of image forming apparatuses (in an electrophotographic type, an ink-jet type, or an offset printing type). In other words, so-called ‘dot gain’ phenomenon may occur.
Due to the dot gain, an image having a darker tone than an intended one may be output, and thus, tone characteristics may degrade. In contrast thereto, with the use of the above-mentioned dot concentration type dither matrix, by which it is possible to increase overlapping of dots as mentioned above, it is possible to cancel out influence of the dot gain. That is, in a unit in which dots are overlapped, spreading of each dot cancels out each other, and thus, adverse influence due to the spreading of each dot does not actually appear. Accordingly, it is possible to achieve image output with a superior tone characteristics, with the use of the dot concentration type dither matrix.
On the order hand, the above-mentioned line screen type dither matrix has the following advantages. In the dot concentration type, in the prior art, a cyclic structure of growth center should be an approximately square. Therefore, merely a small degree of freedom is allowed for the number of screen lines and a screen angle in the dither matrix. In contract thereto, in the line screen type, there occurs no difference even in a case where the cyclic structure of a growth center is a rectangle or a parallelogram from a case where it is a square. Accordingly, it is possible to apply variable combinations of the number of screen lines and a screen angle, i.e., it is possible to increase a selection freedom in this term.
The line screen type dither matrix has advantages also in a case where a plurality of color images are superposed together to form an image such as a color image. When different color images are superposed together, an interference pattern called ‘color moiré’ may appear. In order to solve this problem, the screen angles may be set to be mutually different between the different color screens. In a full-color image, it is required to set a screen angle for each of C (cyan), M (magenta), Y (yellow) and K (black) separately, and it is required to make a setting such that an angle difference therebetween is made as wide as possible, in terms of reduction of the color moiré. In this case, in the line screen type dither matrix, it is possible to set direction axes of these four colors, i.e., CMYK in a range of 180 degrees. Thus, it is possible to make a setting to achieve a wider angle difference between the respective screens of the colors CMYK, in comparison to the case of applying the dot concentration type dither matrix in the prior art. As a result, it is possible to produce an image with a reduced color moiré, easily with the use of the line screen type dither matrix. However, in the case of the line screen type dither matrix, an area where dots are overlapped is smaller than that in the case of the dot concentration type dir matrix. Accordingly, the dot concentration type dither matrix is still advantageous in terms of providing an image of superior tone characteristics.
The Bayer type dither matrix is a dither matrix opposite to the dot concentration type dither matrix, and is a dither matrix for disposing particular dots in a scattering manner as much as possible. Thereby, this type of dither matrix has a function of maintaining a resolution at a high level, while having a demerit that tone reproducibility may not be sufficient. In an earlier or traditional image forming apparatus having a low resolution (up to 300 dpi), the Bayer type dither matrix was used since the resolution can be maintained. However, recently, a resolution of an image forming apparatus is increased (up to 600 dpi, or 1200 dpi) originally, and as a result, it is not necessarily required to maintain a resolution at a high level. Thus, this type dither matrix may not be up-to-date one recently.
As a conventional method for producing such a dither matrix, a binarization pattern producing method may be cited and the method has a basic pattern shape producing step of producing basic pattern shape of a binarization pattern; a turn-on order determination step of determining a turn-on order of pixels configuring the basic pattern; and a rectangular pattern producing step of producing a rectangular pattern, functioning as the binarization pattern, based on the basic patterns (see Japanese Laid-open Patent Application No. 2003-163806, for example).
In the basic matrix shape producing method disclosed by Japanese Laid-open Patent Application No. 2003-163806, although the requirement that basic matrix shape is cyclically disposed is fulfilled, the following problem may occur. That is, according to this prior art, there may occur a case where a growth center position which is a central position of the basic matrix does not coincide with a central position of a pixel. Accordingly, in response to the basic matrix shape selected, the basic matrix central position may vary among four types. Since the basic matrix central position thus may vary, there may occur a problematic situation in which, if a common growth rule is applied to determine a pixel growth order in the basic matrix, a growth order in a halftone dot may not agree with each other. This means that, when the basic matrix shape is changed, such a change is required for changing the number of screen lines or a screen angle of the dither matrix, the dither matrix may be produced in which halftone dots have different growth orders simultaneously.
Furthermore, according to the method of Japanese Laid-open Patent Application No. 2003-163806, it is not possible to determine a growth order only based on a pixel arrangement in the basic matrix. That is, in this prior art, when any pixel within the basic matrix is set as a growth center, a pixel which is located away from this growth center within the relevant basic matrix may be located near a growth center of another basic matrix (the growth center other than the relevant growth center). In such a case, it is not possible to determine the growth order only from information concerning a distance from the growth center of the relevant basic matrix.
Furthermore, the method of Japanese Laid-open Patent Application No. 2003-163806 may not positively avoid the problematic situation in which a pixel located away from a growth center of a relevant basic matrix is located rather near a growth center of another basic matrix, i.e., the growth center other than the relevant growth center. Therefore, the above-mentioned problem may occur in which the selected basic matrix shape may not compatible with the dot concentration type growth order.
As another example, a dither matrix producing method in which a dither matrix is produced based on screen lines employs a dither matrix setting unit determining a core of a dot producing a screen line, a shape of the dot core changing according to a tone level, and a screen angle; and a dot core arrangement producing unit producing a disposing order of the dot cores of a dither matrix in a form of a blue noise mask (see Japanese Laid-open Patent Application No. 2003-259118, for example).
Japanese Laid-open Patent Application No. 2003-259118 mentioned above discloses a method for ranking of basic matrixes themselves, but does not disclose a method for determining the basic matrix shape itself. Further, this prior art document merely mentions, for a pixel growth order within the basic matrix, ‘to spread it from the center’.
As further another example, an image forming apparatus includes a storing unit to store a matrix shift vector indicating a shift direction and a shift amount of a dither matrix as a parameter prescribing the dither matrix; and a processing matrix element calculating unit to obtain matrix elements used for processing a pixel to be processed based on the matrix shift vector and a position of the above-mentioned pixel to be processed (see Japanese Laid-open Patent Application No. 2003-134337, for example).
According to Japanese Laid-open Patent Application No. 2003-134337 mentioned above, a basic matrix shape (a shape of one cycle of a dither matrix), i.e., an area representing a collection of pixels belonging to one growth center is expressed as a collection of pixels belonging to two rectangles. A basic matrix shape may be preferably expressed by such two rectangles for the purpose of expressing the basic matrix shape in a visually easily understandable manner. However, if this method in the prior art is applied for producing the dot concentration type dither matrix, the following problem may occur. That is, it is not possible to determine a growth order only according to a pixel arrangement within a relevant basic matrix. That is, when any pixel within the basic matrix is set as a growth center (i.e., a pixel acting as a center of dot concentration type dither), a pixel located away from this growth center in the relevant basic matrix may be located rather near a growth center of another basic matrix, i.e., the growth center other than the relevant growth center. In such a case, it is not possible to determine the growth order only according to the information of a distance from the growth center of the relevant the basic matrix.
As to a growth order within a basic matrix, Japanese Laid-open Patent Application No. 2003-259118 mentioned above discloses ‘to spread it from the center’, while Japanese Laid-open Patent Application No. 2003-134337 mentioned above discloses ‘a manner such that a length of a boundary contour may become minimum’. However, there is no disclosure in these prior art documents for a specific algorithm for achieving such a manner of growth order. In particular, there is no idea disclosed as how to order pixels having an equal distance from a growth center. Therefore, this matter may become a problem when a computer program is actually produced for desired number of screen lines and screen angle.